when you use centmin.sh menu option 5 to do a php upgrade specify version in format = 7.1.0alpha1 as per snapshot download tarballs at https://downloads.php.net/~krakjoe/ and outlined at https://community.centminmod.com/threads/php-7-1-0-alpha-1.7607/. This will download those source tarballs and install PHP 7.1.0 Alpha 1 or or 7.1.0alpha2, 7.1.0alpha3 when they are announced.

if you specify PHP version = 7.1.0 right now, you will get latest git repo alpha development release.